linux-kernel - Re: [PATCH v6 5/5] vfio/type1: Use mdev bus iommu

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Date:   Tue, 3 Nov 2020 13:22:05 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Cornelia Huck <cohuck@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Zeng, Xin" <xin.zeng@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v6 5/5] vfio/type1: Use mdev bus iommu_ops for IOMMU
 callbacks

Hi Alex,

On 10/31/20 5:06 AM, Alex Williamson wrote:
> On Fri, 30 Oct 2020 06:16:28 +0000
> "Tian, Kevin" <kevin.tian@...el.com> wrote:
> 
>>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>>> Sent: Friday, October 30, 2020 12:58 PM
>>>
>>> With the IOMMU driver registering iommu_ops for the mdev_bus, the
>>> IOMMU
>>> operations on an mdev could be done in the same way as any normal device
>>> (for example, PCI/PCIe). There's no need to distinguish an mdev from
>>> others for iommu operations. Remove the unnecessary code.
>>
>> This is really a nice cleanup as the output of this change! :)
> 
> It's easy to remove a bunch of code when the result is breaking
> everyone else.  Please share with me how SR-IOV backed mdevs continue
> to work on AMD platforms, or how they might work on ARM platforms, when
> siov_iommu_ops (VT-d only) becomes the one and only provider of
> iommu_ops on the mdev bus.  Hard NAK on this series.  Thanks,

I focused too much on a feature and forgot about university. I should
apologize for this. Sorry about it!

Back to the original intention of this series. The aux domain was
allocated in vfio/mdev, but it's also needed by the vDCM component of a
device driver for mediated callbacks. Currently vfio/mdev or iommu core
has no support for this.

We had a proposal when we first did aux-domain support. But was not
discussed since there was no consumer at that time.

https://lore.kernel.org/linux-iommu/20181105073408.21815-7-baolu.lu@linux.intel.com/

Does it look good to you? I can send patches of such solution for
discussion if you think it's a right way.

Extending the iommu core for subdevice passthrough support sounds an
interesting topic, but it will take much time before we reach a
consensus. It sounds a good topic for the next year's LPC/MC :-).

Best regards,
baolu